usually the year before, because it's the year of the class they went to school with. but sometimes a real's bandmates or friends are good indicators and they might still call the older bandmate by an honorific or consider themselves part of their birth year's ageline.

for example 97line has early 97ers but afaik doesn't include any early 98ers. some early 98ers they're close to call some of the 97ers hyung, and others don't. basically do whatever lol unless the real indicates a preference.